That basically works (the style is added to the header element), but the file http://nettra.net/demo-sites/wvg/development/templates/wvgs/css/bootstrap.css sets the background of the header using background: rgba(0,0,0,0.5)!important; at about line #10983, overriding your inline style. If you remove !important, and make the changes above, it should work.

Isn't it the $ sign need to be replaced with 'jQuery' for better compatibility as @johanpw mentioned earlier ?
– webchunSep 4 '15 at 2:09

You see that I have used jQuery(document).ready(function($) {. This means jQuery is the global scope which is what @johanpw mentioned to prevent conflicts, but I have used $ as a local alias which will not cause conflicts as it's locked to only this scope
– LodderSep 4 '15 at 10:17

Yeah, I managed to replace all of the '$' with jQuery, and that seemed to have fixed it. Plus, I had another version of jQuery running (DOH!).
– user1813113Sep 8 '15 at 19:58